This invention relates to ultrasound imaging, more partficularly to novel contrast agent preparations and their use in ultrasound imaging, for example in visualising tissue perfusion.
It is well known that contrast agents comprising dispersions of microbubbles of gases are particularly efficient backscatterers of ultrasound by virtue of the low density and ease of compressibility of the microbubbles. Such microbubble dispersions, if appropriately stabilised, may permit highly effective ultrasound visualisation of, for example, the vascular system and tissue microvasculature, often at advantageously low doses.
The use of ultrasonography to measure blood perfusion (i.e. blood flow per unit of tissue mass) is of potential value in, for example, tumour detection, tumour tissue typically having different vascularity from healthy tissue, and studies of the myocardium, e.g. to detect myocardial infarctions. A problem with the application of existing ultrasound contrast agents to cardiac perfusion studies is that the information content of images obtained is degraded by attenuation caused by contrast agent present in the ventricles of the heart.
The present invention is based on the finding that ultrasonic visualisation of a subject, in particular of perfusion in the myocardium and other tissues, may be achieved and/or enhanced by means of gas-containing contrast agent preparations which promote controllable and temporary growth of the gas phase in vivo following administration. Thus, for example, such contrast agent preparations may be used to promote controllable and temporary retention of the gas phase, for example in the form of microbubbles, in tissue microvasculature, thereby enhancing the concentration of gas in such tissue and accordingly enhancing its echogenicity, e.g. relative to the blood pool.
It will be appreciated that such use of gas as a deposited perfusion tracer differs markedly from existing proposals regarding intravenously administrable microbubble ultrasound contrast agents. Thus it is generally thought necessary to avoid microbubble growth since, if uncontrolled, this may lead to potentially hazardous tissue embolisation. Accordingly it may be necessary to limit the dose administered and/or to use gas mixtures with compositions selected so as to minimise bubble growth in vivo by inhibiting inward diffusion of blood gases into the microbubbles (see e.g. WO-A-9503835 and WO-A-9516467).
In accordance with the present invention, on the other hand, a composition comprising a dispersed gas phase is coadministered with a composition comprising at least one substance which has or is capable of generating a gas or vapour pressure in vivo sufficient to promote controllable growth of the said dispersed gas phase through inward diffusion thereto of molecules of gas or vapour derived from said substance, which for brevity is hereinafter referred to as a xe2x80x9cdiffusible componentxe2x80x9d, although it will be appreciated that transport mechanisms other than diffusion may additionally or alternatively be involved in operation of the invention, as discussed in greater detail hereinafter.
This coadministration of a dispersed gas phase-containing composition and a composition comprising a diffusible component having an appropriate degree of volatility may be contrasted with previous proposals regarding administration of volatile substances alone, e.g. in the form of phase shift colloids as described in WO-A-9416739. Thus the contrast agent preparations of the present invention permit control of factors such as the probability and/or rate of growth of the dispersed gas by selection of appropriate constituents of the coadministered compositions, as described in greater detail hereinafter, whereas administration of the aforementioned phase shift colloids alone may lead to generation of microbubbles which grow uncontrollably and unevenly, possibly to the extent where at least a proportion of the microbubbles may cause potentially dangerous embolisation of, for example, the myocardial vasculature and brain (see e.g. Schwarz, Advances in Echo-Contrast [1994(3)], pp. 48-49).
It has also been found that administration of phase shift colloids alone may not lead to reliable or consistent in vivo volatilisation of the dispersed phase to generate gas or vapour microbubbles. Grayburn et al. in J. Am. Coll. Cardiol. 26(5) [1995], pp. 1340-1347 suggest that preactivation of perfluoropentane emulsions may be required to achieve myocardial opacification in dogs at effective imaging doses low enough to avoid haemodynamic side effects. An activation technique for such colloidal dispersions, involving application of hypobaric forces thereto, is described in WO-A-9640282; typically this involves partially filling a syringe with the emulsion and subsequently forcibly withdrawing and then releasing the plunger of the syringe to generate a transient pressure change which causes formation of gas microbubbles within the emulsion. This is an inherently somewhat cumbersome technique which may fail to give consistent levels of activation.
It is stated in U.S. Pat. No. 5,536,489 that emulsions of water-insoluble gas-forming chemicals such as perfluoropentane may be used as contrast agents for site-specific imaging, the emulsions only generating a significant number of image-enhancing gas microbubbles upon application of ultrasonic energy to a specific location in the body which it is desired to image. Our own research has shown that emulsions of volatile compounds such as 2-methylbutane or perfluoropentane give no detectable echo enhancement either in vitro or in vivo when ultrasonicated at energy levels which are sufficient to give pronounced contrast effects using two component contrast agents in accordance with the present invention.
According to one aspect f the invention there is provided a combined preparation for simultaneous, separate or sequential use as a contrast agent in ultrasound imaging, said preparation comprising:
i) an injectable aqueous medium having gas dispersed therein; and
ii) a composition comprising a diffusible component capable of diffusion in vivo into said dispersed gas so as at least transiently to increase the size thereof.
According to a further aspect of the invention there is provided a method of generating enhanced images of a human or non-human animal subject which comprises the steps of:
i) injecting a physiologically acceptable aqueous medium having gas dispersed therein into the vascular system of said subject;
ii) before, during or after injection of said aqueous medium administering to said subject a composition comprising a diffusible component capable of diffusion in vivo into said dispersed gas so as at least transiently to increase the size thereof; and
iii) generating an ultrasound image of at least a part of said subject.
This method according to the invention may advantageously be employed in visualising tissue perfusion in a subject, the increase in size of the dispersed gas being utilised to effect enrichment or temporary retention of gas in the microvasculature of such tissue, thereby enhancing its echogenicity.